This invention relates in general to electrostatography and, more specifically, to a novel photoconductive device and processes for preparing and using the device.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind an electrostatic latent image in the nonilluminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may also function as an anode when the charge transport layer is sandwiched between the anode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain diamine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed, for example, in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. However, when the supporting conductive substrate comprises a metal having an outer oxide surface such as aluminum oxide, difficulties have been encountered with these photosensitive members under extended electrostatographic cycling conditions found in high volume, high speed copiers, duplicators and printers. For example, it has been found that when certain charge generation layers comprising a resin and a particulate photoconductor are adjacent an aluminum oxide layer of an aluminum electrode, the phenomenon of "cycling-up" is encountered. Cycling-up is the build-up of residual potential through repeated electrophotographic cycling. Build-up of residual potential can gradually increase under extended cycling to as high, for example, as 300 volts. Residual potential causes the surface voltage to increase accordingly. Build-up of residual potential and surface voltage causes ghosting, increased background on final copies and cannot be tolerated in precision high-speed, high-volume copiers, duplicators, and printers.
It has also been found that photosensitive members having a homogeneous generator layer such as As.sub.2 Se.sub.3 such as those disclosed in U.S. Pat. No. 4,265,990, exhibit "cycling-down" of surface voltage when exposed to high cycling conditions found in high speed, high volume copiers, duplicators and printers. When cycling-down occurs the surface voltage and charge acceptance decrease as the dark decay increases in the areas exposed and the contrast potential for good images degrades and causes faded images. This is an undesirable fatigue-like problem and is unacceptable for high speed, high volume applications.
The use of electrical blocking layers has reduced the "cycling-up" and "cycling-down" problems. One example is the use of siloxanes as described, for example in U.S. Pat. No. 4,464,450 to L. Teuscher, issued August 7, 1984. Excellent images may be obtained with the siloxanes described in U.S. Pat. No. 4,464,450. However, the siloxane film cannot be cast thick as required for some applications.
Copolymers of methyl vinyl ether and maleic anhydride such as the Gantrez AN resins from General Aniline & Film Corporation have been utilized in blocking layers. Unfortunately, these copolymers of methyl vinyl ether and maleic anhydride are sensitive to water and rapidly hydrolyze to form acidic products which are corrosive and attack metal ground planes of photoreceptors during cycling. Loss of the ground plane due to corrosion during cycling eventually prevents an electrophotographic imaging member from discharging. This is manifested by an increase in the background deposits in the final image during cycling. In addition, the mechanical properties of copolymers of methyl vinyl ether and maleic anhydride are affected at high humidity and cause flexible electrophotographic imaging members to delaminate. Under low humidity conditions, blocking layers containing copolymers of methyl vinyl ether and maleic anhydride or maleic anhydride tends to cycle down electrical surface potential. Cycle down affects the final copy by loss of contrast between exposed and unexposed areas. In addition, copolymers of methyl vinyl ether and maleic anhydride are sensitive to certain solvents utilized in subsequently applied layers and white spots may form in the final image. White spots are defined as electrical potential deficient spots. Hydrolysis of copolymers of methyl vinyl ether and maleic anhydride transforms the anhydride to the acid. The acid formed during storage will attack the metallic conductive layer and result in photoreceptors that will no longer discharge. Moreover, during cycling, corrosion of thin metal ground planes is accelerated and this will also result in photoreceptors that will no longer discharge. Also, when the acid is formed, coating with the material is restricted to coating with water and low molecular weight alcohols.